This invention relates to X-ray diffraction systems. X-ray diffraction is a non-destructive technique for the qualitative and quantitative analysis of crystalline material samples, which are generally provided in the form of crystals or powders. In accordance with this technique, an X-ray beam is generated by an X-ray tube with a stationary anode, by a conventional rotating anode X-ray source or by a synchrotron source and directed toward the material sample under investigation. When the X-rays strike the sample, they are diffracted according to the atomic structure of the sample.
A typical laboratory system 100 for performing single crystal diffraction experiments normally consists of five components as shown in FIG. 1. The components include an X-ray source 102 that produces a primary X-ray beam 104 with the required radiation energy, focal spot size and intensity. X-ray optics 106 are provided to condition the primary X-ray beam 104 to a conditioned, or incident, beam 108 with the required wavelength, beam focus size, beam profile and divergence. A goniometer and stage 110 are used to establish and manipulate geometric relationships between the incident X-ray beam 108, the crystal sample 112 and the X-ray detector 114. The incident X-ray beam 108 strikes the crystal sample 112 and produces scattered X-rays 116 which are recorded in the detector 114. A sample alignment and monitor assembly comprises a sample illuminator 118, typically a laser, that illuminates the sample 112 and a sample monitor 120, typically a video camera, which generates a video image of the sample to assist users in positioning the sample in the instrument center and monitoring the sample state and position.
In order to increase the X-ray intensity at the crystal sample, focusing optics are routinely used. The X-ray beam path 200 of a typical single crystal diffraction set-up is schematically shown in FIG. 2. In this set-up, a multi-layer focusing X-ray optic 202 is used. Optic 202 has a focusing surface that is part of an ellipse, schematically shown as 204 in FIG. 2. An X-ray source 206 is placed at a first focal point of optic 202. This source generates X-rays 208 that are redirected by optic 202 to form a focused and redirected beam 210, which is focused on an image 212 at the second focal point. The sample is placed at the position of the image 212. The source and image focal points are located at distances f1 and f2 from the middle of the optic 202, respectively.
Most applications require an X-ray beam that is large enough to completely illuminate the sample. Single crystal diffraction systems are thus designed to produce an image at the sample location with a size, or diameter, that is comparable in size to the dimension of a typical sample, which is a few tenths of a millimeter. However, some applications require much smaller image sizes. For example, in some applications, diffraction data produced from only a part of the sample must be obtained. Such local analysis could be needed if the sample contains different parts with different properties. To study these parts separately, the sample must be illuminated with a beam image smaller than the sample. In particular, a method to make a beam with an image size smaller than 0.1 mm is thus desired.
Traditionally, X-ray image size is made smaller by placing apertures in-between the X-ray optic and image focal point. This is routinely done in commercial diffraction equipment, such as the X8 Proteum X-ray diffraction system manufactured and sold by Bruker AXS Inc., Madison, Wis. In the simplest case, one aperture is placed close to the image focal point, as shown in FIG. 3. In this set-up 300, aperture 214 has been added to the set-up illustrated in FIG. 2. The resulting beam 218 is smaller than the original beam (shown as dotted lines 216). The beam image can be made smaller and smaller by using smaller and smaller apertures, but there is a limit. For an infinitely small aperture the image size of the image 212 is determined by the distance (x) between the aperture 214 and the image focal point times the beam divergence.
Ray-tracing illustrates the limitations of an aperture to reduce the image size. The X-ray flux and beam size (at the image focal point) were calculated for a range of aperture diameters and are shown in FIG. 4 for two aperture distances: x=20 mm and x=30 mm. In these calculations, the flux numbers have been normalized. In the calculations, an optic with f1/f2=100 mm/300 mm, a divergence of 4.5 mrad and a source diameter of 0.1 mm were assumed. These parameters are typical for protein single crystal diffraction systems and represent a realistic situation. The results show that an aperture is capable of reducing the image size, but only for large sized beams. For smaller sized beams, the flux quickly reduces and image sizes beyond a minimum size are not practical because the flux is so low.
Positioning the aperture closer to the sample extends the range of image sizes, but this range is also limited because of practical reasons. Some space between aperture and sample must be kept clear, for example, to accommodate an additional aperture to block scattered radiation or to allow for handling of the sample. Consequently, the aperture cannot be placed exactly at the sample (x=0). Accordingly, there is a limit on the smallest image size.
The image size can be reduced further by the introduction of a second aperture, as illustrated in the arrangement 500 shown in FIG. 5. This aperture 220, placed close to the X-ray optic 202, reduces the divergence and thus the intensity of the beam 210. With a reduced divergence, the aperture 214 close to the image area 212 is much more capable of reducing the image size.
FIG. 6 shows ray-tracing results of the arrangement shown in FIG. 5, with two optimized apertures using the same parameters as in FIG. 4. As can be seen in FIG. 6, the introduction of a second aperture does enable the generation of smaller sized beams, but the beam flux quickly diminishes. In the ideal case, a one-aperture system reduces only the size and not the intensity of the beam whereas a two-aperture system reduces both the size and intensity. The end effect of the combined apertures is that the flux in a small sized beam is not reduced by the square of the reduction in the beam diameter (as would be the ideal case shown by the dotted line in FIG. 6), but by the fourth power. This quickly leads to a flux too small to be useful in case of beam images smaller than 0.1 mm.
Another conventional method for producing a small image size is to use a small source size. For example, in FIG. 2, the optic 202 magnifies the source 206 with a magnification factor M=f2/f1. If the source 206 has a size S the resulting beam image A at the sample 212 is then M times S (A=MS). Consequently, the beam image size can be made small by reducing the source size, S and using a low magnification, M. In one prior art device, a micro-focus tube is combined with a focusing optic with a magnification of one. The small source of a micro-focus tube produces a small beam, but the X-ray beam brightness at the sample is only slightly larger than a conventional sealed tube using apertures. Consequently, a method to make beams that are both small and intense is thus desired.